1. Field of the Invention
The present invention relates to a liquid crystal display for displaying images.
2. Description of the Related Art
A liquid crystal display is a non-emission type display for displaying images by adjusting an optical transmission amount and has characteristics of thinness, light weight and low consumption power.
Liquid crystal displays include a transmission type liquid crystal display and a reflection type liquid crystal display. In the transmission type liquid crystal display, a light source (hereinafter called a backlight) is disposed at the back of the display and an optical transmission amount of the backlight is adjusted to display images. In the reflection type liquid crystal display, external light such as room illumination and sun light is made incident upon the display from the front side and the optical reflection amount of the external light is adjusted to display images. There is a liquid crystal display (hereinafter called a transflective type liquid crystal display) which can be used as a reflection type liquid crystal display in a bright environment and as a transmission type liquid crystal display in a dark environment. The transflective type liquid crystal display has both display functions of the reflection type and transmission type, and can reduce a consumption power by turning off a backlight in the bright environment. In the dark environment, images become visible by turning on the backlight. Namely, this liquid crystal display is suitable for a liquid crystal display of a portable apparatus such as a portable phone and a digital camera which is supposed to be in various illumination environments.
One of driving modes realizing a liquid crystal display is a vertical alignment (VA) mode. Liquid crystal molecules used in the VA mode are aligned perpendicular to substrate surfaces while voltage is not applied. A pair of substrates is sandwiched between two polarization plates whose absorption axes cross at a right angle (hereinafter, this state is called cross-nicol) so that light of the backlight is intercepted by the polarization plates to present black display.
A viewing angle range during black display of a liquid crystal display in the VA mode can be broadened by disposing a retardation film called a negative C plate (hereinafter called a negative C-Plate) between the liquid crystal layer and the polarization plate. The negative C-Plate is a negative index ellipsoid which has a refractive index of almost 0 in an in-plane direction and whose refractive index in a thickness direction is smaller than the refractive index in the in-plane direction. The liquid crystal display in the VA mode adopting the negative C-Plate is therefore applied to a large size liquid crystal television which is required to have a large viewing angle.
The liquid crystal display in the VA mode adopts a multi-domain to provide a plurality of tilt directions of liquid crystal molecules and to improve color change, gradation reversal and the like during gray scale display. This can be achieved by alignment control projections, electrode slits and the like. For example, it is possible to tilt liquid crystal molecules omnidirectionally be disposing a circular projection in a pixel center area. However, if liquid crystal molecules are tilted omnidirectionally, a black line called a domain is observed in a cross-nicol state in the area where liquid crystal molecules are tilted along a direction parallel to an absorption axis of each polarization plate and light cannot be transmitted. This may cause a lowered transmittance.
In order to avoid this, the liquid crystal display in the VA mode applied to a large size liquid crystal television has an electrode structure which makes liquid crystal molecules tilt in approximately a 45° direction relative to the absorption axis of the polarization plate. However, this electrode structure lowers an aperture ratio more than the above-described circular projection is disposed in the pixel center area. As a method of solving this issue, JP-A-5-113561 (which corresponds to EP 0538796A1) discloses a method of utilizing a circular polarization plate combining a polarization plate and a quarter wavelength plate. In this case, light transmits even in the domain region so that a high transmittance can be achieved.
In practical use, a large bandwidth circular polarization plate is adopted applying a quarter wavelength plate having low wavelength dependency. As described in JP-A-10-68816, the large bandwidth circular polarization plate can be realized by combining a polarization plate, a half wavelength plate and a quarter wavelength plate.
By applying a circular polarization plate to a liquid crystal display in the VA mode, a trnasflective type liquid crystal display can be realized as described in, for example, JP-A-2003-295165 (which corresponds to U.S. 2003/0160928A1).
As described above, the liquid crystal display in the VA mode using a circular polarization plate can effectively utilize the domain region in a pixel. It is also possible to realize a transflective type liquid crystal display suitable for portable apparatus. However, with the liquid crystal display in the VA mode adopting a circular polarization plate, a contrast ratio lowers because optical leak during black display appearing as observed along an oblique direction cannot be suppressed sufficiently even a negative C-plate is used. Namely, there arises an issue that the liquid crystal display in the VA mode using a circular polarization plate cannot obtain sufficient viewing angle characteristics.